Reflective electrophoretic display with stacked color cells

ABSTRACT

A reflective color electrophoretic display intended to be viewed while illuminated from the front window by ambient light and to operate without the need of a backlight has a plurality of laterally adjacent picture elements or pixels. Each pixel is comprised of two or more subpixels, or cells, which are vertically stacked, one directly above the other on the horizontal surface of a reflective panel located at the rear or bottom of the stacks. The cells contain a light-transmissive fluid and charged pigment particles that can absorb a portion of the visible spectrum, with each cell in a stack containing particles having a color different from the colors of the particles in the other cells in the stack.

RELATED APPLICATION

This application is related to concurrently filed application titled“Transmissive Electrophoretic Display With Stacked Color Cells”. U.S.application Ser. No. 09/860,265 filed on May 18, 2001.

TECHNICAL FIELD

The present invention relates to electrophoretic cells that form anelectrophoretic display. In particular the invention relates to astacked cell configuration for use in a color electrophoretic displayoperating in a light-reflective mode.

BACKGROUND OF THE INVENTION

An electrophoretic cell is a cell comprised of pigment particlessuspended in a fluid and uses electrophoresis to switch between thefollowing two states:

Distributed State: Particles are positioned to cover the horizontal areaof the cell. This can be accomplished, for example, by dispersing theparticles throughout the cell, by forcing the particles to form a layeron the horizontal surfaces of the cell, or by some combination of both.

Collected State: Particles are positioned to minimize their coverage ofthe horizontal area of the cell, thus allowing light to be transmittedthrough the cell. This can be accomplished, for example, by compactingthe particles in a horizontal area that is much smaller than thehorizontal area of the cell, by forcing the particles to form a layer onthe vertical surfaces of the cell, or by some combination of both.

The electrophoretic cell can serve as a light valve since thedistributed and collected states can be made to have different lightabsorbing and/or light scattering characteristics. As a result, anelectrophoretic cell can be placed in the light path between a lightsource and a viewer and can be used to regulate the appearance of apicture element or “pixel” in a display. The basic operation ofreflective electrophoretic cells along with the examples of variouselectrode arrangements are described in IBM's U.S. Pat. Nos. 5,745,094and 5,875,552.

Reflective color displays are known that use liquid crystals inconjunction with a fixed polarizer element to control the intensity oflight reflected form each pixel. Since polarizers absorb the fraction oflight whose polarization is not aligned with their active axis, andsince this absorption varies with the angle of incidence, displays basedon their use suffer from both limited reflectivity and viewing angle.

Other reflective color displays are known that use a solution of adichroic dye in single or multiple layers of either a nematic orcholesteric liquid crystal material. Using a single nematic layersrequires the use of a fixed polarizer element and therefore suffers fromthe aforementioned limitations. Using one or more cholesteric layers, ormore than one nematic layer, eliminates the need for a fixed polarizerelement and increases the achievable reflectivity. This approach stillrelies on the selective absorption of polarized light and, as a result,the contrast changes with viewing angle.

Other reflective color displays are known that use scattering liquidcrystal materials, such as polymer-dispersed liquid crystal materials orscattering-mode polymer stabilized cholesteric texture materials, tocontrol the intensity of light reflected from each pixel by switchingbetween a turbid state and a uniform state. Since these materials onlyweakly scatter light in their turbid state, reflective displays based onthem have a low diffuse reflectance and therefore also suffer from lowbrightness.

Other reflective color displays are known that use reflecting liquidcrystal materials, such as reflective-mode polymer-stabilizedcholesteric texture materials or holographic-polymer-dispersed liquidcrystals, to control both the intensity and color or reflected lightreflected from each pixel via diffraction effects. Since these depend ondiffraction effects, it is difficult to simultaneously achieve largeviewing angle, high reflectance, and angle independent color.

Liquid crystal displays commonly use a side-by-side arrangement ofsingle color subpixels within each pixel to generate color via spatialcolor synthesis. The reflection efficiency of such an arrangement islimited by the fact that each subpixel only occupies a fraction of thetotal pixel area. By arranging the subpixels in a vertical stack, eachsubpixel can occupy the same lateral area as the pixel itself, and thereflection efficiency can be significantly increased. U.S. Pat. No.5,625,474 assigned to the Sharp Corporation and IBM's U.S. Pat. No.5,801,796 describe embodiments of stacked cell arrangements suitable forliquid crystal materials. Since these embodiments rely on liquid crystalmaterials, however, they remain hindered by the aforementionedlimitations of such materials. Electrophoretic displays do not sufferfrom these limitations and can offer improved reflection characteristicscombined with extremely low power requirements.

Reflective color electrophoretic displays have been proposed in theprior art. Japanese Patent JP 1267525 assigned to Toyota MotorCorporation describes an electrophoretic display having colored (blueand yellow) particles with different zeta potentials in a solution ofred dye to give a multicolored (yellow, green and red) display. When acertain voltage is applied to the pixels, the yellow particles arepulled to the front transparent electrode and the viewer sees yellow. Ata higher voltage, the blue particles are also pulled to the frontelectrode and the viewer sees green. When the particles are pulled offthe transparent electrode, the colors of the particles are hidden by thedye solution and the viewer sees red.

U.S. Pat. No. 3,612,758 assigned to Xerox Corporation describes areflective electrophoretic display having pigment particles of a singlecolor in a contrasting dye solution. In this scheme, under the influenceof an electric field, the particles migrate to a front transparentelectrode and the viewer sees the color of the particles. When the fieldis reversed, the particles migrate away from the front transparentelectrode, are hidden in the dye solution, and the viewer sees the colorof the dye solution.

In the two electrophoretic display patents above, color contrast andreflectance depend on the presence or absence or particles at the frontwindow. Since the dye solution can not be completely removed from thespace between the particles when they are at the front window, displaysbased on this approach do not produce high contrasted images andgenerally have a low reflectance.

WO 94/28202 assigned to Copytele Inc. describes a dispersion for areflective electrophoretic display comprised of two differently coloredparticles that are oppositely charged. The polarity of the voltageapplied to the cell determines the polarity of the particle attracted tothe front transparent electrode, and hence determines the color seen bythe viewer. Since the viewer sees either one of two colors, thisapproach produces monochrome images and therefore has a limited colorgamut.

U.S. Pat. No. 5,276,438 assigned to Copytele Inc. describes a reflectiveelectrophoretic display in which a mesh screen, disposed behind thefront window and covering the viewing area of the display, is usedeither with or without a dyed suspension fluid to hide particles of asingle color from the viewer. When the particles are positioned in frontof the mesh, the viewer sees the color of the particles. When theparticles are positioned behind the mesh, the viewer sees a mixture ofthe mesh and particle colors. As a result, the contrast produced by thisapproach is limited by the open area of the mesh. In addition, thisapproach produces monochrome images and therefore has a limited colorgamut.

U.S. Pat. No. 3,668,106 assigned to Matsushita Electric describesreflective electrophoretic displays using white or coloredelectrophoretic particles in colored or transparent suspension media ina side-by-side arrangement of colored subpixels to generate color viaspatial color synthesis. IBM's U.S. Pat. No. 6,225,971 and pendingapplication 09/154,284 also describe reflective electrophoretic displaysthat rely on spatial color synthesis, but which have improvedreflectivity and color gamut. As stated above, however, the reflectionefficiency of color generation via spatial color synthesis is limited bythe fact that each subpixel only occupies a fraction of the total pixelarea.

There is a continuing need in the art for a low-power reflective colordisplay with high reflectance, high image contrast, and large colorgamut. It would be desirable, therefore, to incorporate the advantagesoffered by electrophoresis in a scheme that can utilize verticallystacked subpixels to maximize reflection efficiency. Electrophoreticdisplays that rely on hiding particles in a dye or behind a mesh are notsuitable for stacking, since their reflectivity and contrast originatefrom the need to prevent light from passing through both the particlesand the hiding medium. Furthermore, stacked cell structures suitable forliquid crystal materials are not appropriate for stackableelectrophoretic schemes. In particular, the lateral parallel-plateelectrode geometries used in both stacked and non-stacked liquid crystaldisplays are not capable of switching an electrophoretic suspensionbetween its distributed and collected states. In addition, sinceelectrophoretic suspensions can be influenced by weak electric fields,such geometries do not provide sufficient isolation of any givensubpixel from the stray electric fields that originate from itsneighbors.

SUMMARY OF THE INVENTION

The present invention is a reflective color electrophoretic display. Thedisplay is intended to be viewed while illuminated from the front windowby ambient light and to operate without the need of a backlight. Thedisplay is comprised of many picture elements or pixels located inlateral adjacency in a plane. Each pixel is comprised of two or moresubpixels, or cells, which are vertically stacked, one directly abovethe other on a reflective panel located at the rear or bottom of thestacks. The cells contain a light-transmissive fluid and charged pigmentparticles that can absorb a portion of the visible spectrum, with eachcell in a stack containing particles having a color different from thecolors of the particles in the other cells in the stack. The color of apixel is determined by the portion of the visible spectrum that survivesthe cumulative effect of traversing each cell in the stack andreflecting off the reflective panel. Each cell is comprised oflight-transmissive front and rear windows, at least one non-obstructingcounter electrode, and at least one non-obstructing collectingelectrode. A plurality of vertical side walls extend from the rear paneland support the windows in a spaced apart relationship. The side wallsare vertically aligned with one another and thus divide the display intoa plurality of vertical stacks of cells, each stack forming a pixel. Theelectrodes are controlled by solid state switches or driving elements,such as a thin film transistor or a metal-insulator-metal device, formedon the inside surface of the rear panel, with electrical connectionbeing made vertically through holes in the windows that separate thecells in the stacks.

The amount and color of the light reflected by each cell is controlledby the position and the color of the pigment particles within the cell.The position, in turn, is directed by the application of appropriatevoltages to the collecting and counter electrodes. When the pigmentparticles are positioned in the path of the light that enters the cell,the particles absorb a selected portion of this light and the remaininglight is transmitted through the cell. When the pigment particles aresubstantially removed from the path of the light entering the cell, thelight can pass through the cell, reflect from the reflective panel, passthrough the cell again, and emerge without significant visible change.The light seen by the viewer, therefore, depends on the distribution ofparticles in each of the cells in the vertical stacks. Since each of thecells or subpixels in the stack occupy the same lateral area as thepixel itself, the reflection efficiency can be significantly higher thanthat of embodiments that rely on a side-by-side arrangement of subpixelsto generate color.

A more thorough disclosure of the present invention is presented in thedetailed description that follows and from the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side sectional view of one pixel of the display of thepresent invention, illustrating the three stacked cells with theelectrophoretic pigment particles in all the cells in the collectedstate.

FIG. 2 is a side sectional view of one pixel of the display of thepresent invention, illustrating the three stacked cells with theelectrophoretic pigment particles in all the cells in the distributedstate.

FIGS. 3-8 are side sectional views of one pixel of the display of thepresent invention showing the three stacked cells with theelectrophoretic pigment particles in particular cells being in eitherthe collected or distributed state, to thereby illustrate the manner inwhich the pixel achieves different colors.

FIG. 9 is a side sectional view of one pixel of the display of thepresent invention illustrating the collecting and counter electrodes andthe electrical connection from the electrode driving elements to theelectrodes.

FIGS. 10-12 are top views of the pixel illustrating steps in thefabrication of the cells in the stack.

DETAILED DESCRIPTION OF THE INVENTION

The suspension used in the electrophoretic display of the presentinvention is minimally comprised of pigment particles and alight-transmissive fluid. The suspension is preferably highly stablewith both time and use. The suspension is preferably highly resistant toagglomeration, flocculation, and sticking to the surfaces in the cell,even after being compacted and re-dispersed many times. The suspensionpreferably doesn't react with the surfaces in the cell. The specificgravity of the pigment particles and the fluid are preferably similar.The pigment particles preferably acquire a single polarity when placedin suspension.

Optionally, other components may be added to the suspension such ascharge control additives, dispersants, and surfactants to improve theperformance of the suspension. Suitable additives include sodiumdioctylsulfosuccinate, zirconium octoate, and metal soaps such aslecithan, barium petronate, calcium petronate, alkyl succinimide, ironnaphthenate, and polyethylene glycol sorbitan stearate.

The suspension fluid is preferably colorless. The fluid preferably hasminimum solvent action on the pigments and does not react with thesurfaces in the cell. The fluid is preferably dielectric andsubstantially free of ions. The fluid preferably has a low viscosity.The fluid can be a mixture of fluids. Suitable fluids include siliconefluids such as hexamethyldisiloxane, octamethyltrisiloxane,decamethyltetrasiloxane, and other poly(dimethylsiloxane)s. Suitablefluids also include hydrocarbons such as decane, dodecane, tetradecane,xylene, Sohio odorless solvent (a kerosene fraction available from ExxonCompany), toluene, hexane and Isopar® C, E, G, H, K, L, M, and V andNorpar® 12, 13, and 15 (branched and linear saturated aliphatichydrocarbons available from Exxon Company).

The pigment particles can be black, white (reflective) or colored.Suitable colors include cyan, magenta, yellow, red, green, blue, or thelike. Suitable classes of inorganic pigments include:

Cadmium Red

Cadmium sulfo-selenide (black)

Carbon Black

Chromium oxide (green)

Iron oxides (black)

Iron oxides (red)

Lead chromate (yellow)

Manganese dioxide (brown)

Silicon monoxide (reddish brown)

Sulfur (yellow)

Vermilion Red

Suitable classes of organic pigments include:

Anthracene (fluorescent blue, fluorescent yellow)

Anthraquinone (blue, red, yellow)

Azonaphthols (magenta)

Azopyridone (yellow)

Heterocyclic Azo (cyan, magenta)

Methine (yellow)

Nigrosines (black)

Phthalocyanine (blue, green, cyan)

Quinacridone (magenta)

Suitable opaque pigment particles include:

Anric Brown (C.I. Pigment Brown 6)

Cabot Mogul L (black)

C.I. Direct Yellow 86

C.I. Direct Blue 199 (cyan)

C.I. Food Black 2

Dalama® Yellow (Pigment Yellow 74)

Hansa® Yellow (Pigment Yellow 98)

Indo® Brilliant Scarlet (Pigment Red 123)

Monastral® Green G (C.I. Pigment Green 7)

Monastral® Blue B (C.I. Pigment Blue 15)

Monastral® Blue G (C.I. Pigment Blue 15)

Monastral® Green B (C.I. Pigment Green 7)

Paliotol® Black L0080 (C.I. Pigment Black 1)

Permanent Rubine F6BI3-1731 (Pigment Red 184)

Pigment Scarlet (C.I. Pigment Red 60)

Quindo® Magenta (Pigment Red 122)

Stirling NS N 77Y (Pigment Black 7)

Toluidine Red B (C.I. Pigment Red 3)

Toluidine Red Y (C.I. Pigment Red 3)

Toluidine Yellow G (C.I. Pigment Yellow)

Watchung® Red B (C.I. Pigment Red 48)

Other suitable pigment particles will be known to those skilled in theart, such as those described in U.S. Pat. Nos. 5,200,289 and 4,631,244relating to liquid toners for electrophotography, and in IBM's U.S. Pat.No. 5,914,806 for stable electrophoretic particles.

The light-reflective panel located at the rear or bottom of the stackscan be colored or colorless provided it reflects the colors of lighttransmitted by the particles. The panel can be mirrored to reflect allvisible wavelengths (e.g. coated with aluminum, chromium, or silver), ormirrored to reflect selected visible wavelengths (e.g. coated with adielectric stack). Alternatively, the panel can be pigmented white orcan be colored with either a pigment or a dye. The reflected light ispreferably diffused either by the reflective panel itself or by aseparate element. Alternatively, the panel can be replaced byelectrophoretic cells containing reflecting particles.

The collecting and counter electrodes in each cell are constituted orsized or positioned to be non-obstructing. This means that in thecollected state, neither the particle coated collecting electrode northe counter electrode unacceptably interferes with the passage of thedesired color of light as it travels through the cell, i.e.substantially all of the incident light of the desired color is passedthrough the cell.

A non-obstructing collecting electrode can be realized by allowing it tooccupy only a small fraction of the horizontal area of the cell by, forexample, forming it into a narrow line or a small pedestal. It can alsobe realized by disposing it along a vertical wall in the cell. Anon-obstructing counter electrode can be realized similarly or,alternatively, by coating the inside surface of the front window or therear panel with a layer of conductive, light-transmissive material suchas indium tin oxide. Alternatively, a rear panel with a light reflectingmetallic surface can also serve as the counter electrode on thebottom-most cell as well as the reflecting surface.

There can be one or more non-obstructing collecting electrodes and oneor more non-obstructing counter electrodes in each cell and eitherelectrode can be common to more than one cell. They can be disposedvertically and/or horizontally in the cell. The electrodes arepreferably good conductors (e.g. aluminum, chromium, copper, nickel) andcan be light-transmissive (e.g. indium tin oxide). They can be formedentirely of metal or as an electrically conducting film deposited on theappropriate portion of a nonconductive surface. The various electrodearrangements for electrophoretic cells described in IBM's U.S. Pat. Nos.5,872,552 and 6,144,361, which are incorporated herein by reference,will function in the stacked color electrophoretic display of thepresent invention.

The following examples are detailed descriptions of displays of thepresent invention. The details fall within the scope of, and serve toexemplify, the more general description set forth above. The examplesare presented for illustrative purposes only, and are not intended as arestriction on the scope of the invention.

FIGS. 1 through 8 illustrate a preferred embodiment of electrophoreticdisplay cells in accordance with the present invention. Each cell 14,15, and 16 generally comprises a front light{-}transmissive and rearlight-transmissive window. The front light-transmissive window 2 a ofcell 14 and the rear light-transmissive window 4 c of cell 16 also serveas the top and bottom surface of the pixel 26. The front window 2 b ofcell 15 is a thin light-transmissive plate that preferably also servesas the rear window 4 a of cell 14 and vice versa. Likewise, the frontwindow 2 c of cell 16 is a thin light-transmissive plate that preferablyalso serves as the rear window 4 b of cell 15 and vice versa. The rearwindow 4 c of the bottom-most cell is backed by a reflective layer 6.Preferably rear window 4 c is glass with a reflective layer 6 depositedon its outer surface, and thus window 4 c serves as a light-reflectiverear panel for the display.

Each cell 14, 15, and 16 has one or more non-obstructing counter postelectrodes 20 a, 20 b, and 20 c and collecting wall electrodes 8 a, 8 b,and 8 c disposed within the cell. Each cell 14, 15, and 16 also has asuspension comprised of charged pigment particles 10 a, 10 b, and 10 crespectively, in light-transmissive fluids 12 a, 12 b, and 12 crespectively, in the space between its respective front and rearwindows, 2 a and 4 a, 2 b and 4 b, and 2 c and 4 c. The verticallystacked cells 14, 15, and 16 form a reflective electrophoretic colorpixel 26. Only one pixel is illustrated, but the display comprises alarge plurality of laterally adjacent pixels, with adjacent pixelssharing side walls. The side walls 8 a, 8 b, 8 c, which also serve asthe collecting electrodes, support the plates and windows in aspaced-apart relationship. The side walls 8 a, 8 b, 8 c are verticallyaligned above the horizontal surface of rear window 4 c, and thus dividethe display into the plurality of individual color pixels 26. Each cellin a stack also has laterally adjacent like cells which together form alayer of cells in the display. In the preferred embodiment all the cellsin a layer have pigment particles of the same color.

The counter electrodes 20 a, 20 b, and 20 c represent individuallyaddressable posts. The electrical connection to these posts may be madevia vertical wires through the appropriate underlying windows and cells.The horizontal area occupied by a post is much smaller than thehorizontal area of the cell. The collecting electrodes 8 a, 8 b, and 8 calso serve as the thin vertical side walls oriented perpendicularly toboth the front window 2 a, the thin plates that serve as windows 2 b and2 c, and the rear panel with window 4 c, respectively. The counter andcollecting electrodes can be formed entirely of electrically conductivemetal, such as by electrodeposition into a pattern formed in a layer ofphotoresist, followed by removal of the photoresist. The collectingelectrodes 8 a, 8 b, 8 c may also be formed as electrically conductivefilms deposited on the cell-interior surfaces of the nonconductive sidewalls. The four vertical side walls define the perimeter of each cell14, 15, and 16. Laterally or horizontally adjacent pixels share a wall.The side walls that surround each cell form a common structure that isheld at the same voltage. The horizontal area occupied by the side wallsis preferably much smaller than the viewing area of the display.

By appropriately changing the voltage applied to the addressable centerpost electrodes 20 a, 20 b, and 20 c, the cells can be switched betweentheir collected and distributed states. The collected state of the cells14, 15, and 16 are illustrated as one in which the particles haveaccumulated on their respective collecting electrodes 8 a, 8 b, and 8 c.FIG. 1, for example, shows the three cells 14, 15, and 16 in theircollected state. In their collected state, the particles form a thinlayer on a vertically disposed surface and therefore occupysubstantially no horizontal area within a cell. Since the collectionarea of the collection electrode is large, i.e., the entire interiorsurface of the perimeter walls, the particles are not forced into asmall, highly compacted volume. The distributed state of a cell isillustrated as one in which the respective particles 10 a, 10 b, and 10c are generally uniformly dispersed throughout their respectivesuspension fluids 12 a, 12 b, and 12 c. FIG. 2, for example, shows thethree cells 14, 15, and 16 in their distributed state. In theirdistributed state, the particles are disbursed substantially over theentire horizontal area of a cell.

The color of a pixel is determined by the state of, and particle colorassociated with, each cell in its vertical stack. FIGS. 1 through 8illustrate a preferred embodiment that utilizes particles withsubtractive primary colors. The particles 10 a in cell 14 are yellow,the particles 10 b in cell 15 are cyan, the particles 10 c in cell 16are magenta, and the color of the reflective panel 6 is white.

In FIG. 1, the cells 14, 15, and 16 are each in their collected state.Since the particles 10 a, 10 b, and 10 c occupy substantially nohorizontal area in their respective cells, light can pass through eachof the cells without significantly interacting with their respectiveparticles. Ambient light, therefore, will pass through thelight-transmissive front and rear windows of each cell withoutsignificant visible change, reflect off the reflective panel 6, and thenpass back through the light-transmissive rear and front windows of eachcell without significant visible change. As a result, the pixel 26 inFIG. 1 will appear white to the viewer.

In FIG. 2, the cells 14, 15, and 16 are each in their distributed state.Since the particles 10 a, 10 b, and 10 c substantially cover the entirehorizontal area in their respective cells, light passing through thesecells significantly interacts with the particles in each cell. Ambientlight illuminating the cell stack interacts first with the yellowparticles 10 a in cell 14, then with the cyan particles 10 b in cell 15,then with the magenta particles 10 c in cell 16 before reflecting offthe reflecting panel 6, followed by a second interaction with themagenta particles 10 c in cell 16, then with the cyan particles 10 b incell 15, and finally with the yellow particles 10 a in cell 14. As aresult of interacting with all three sets of particles, each possessinga different subtractive primary color, significantly all the ambientvisible light is absorbed in the vertical cell stack and the pixel 26 inFIG. 2 will appear dark or black to the viewer.

In FIG. 3, cell 16 is in its collected state, while cells 14 and 15 areeach in their distributed state. Since the particles 10 c occupysubstantially no horizontal area in cell 16, light can pass through thiscell without significantly interacting with its particles. Since theparticles 10 a and 10 b substantially cover the entire horizontal areain their respective cells, light passing through these cellssignificantly interacts with the particles in each cell. Ambient lightwill therefore pass through cell 16 without significant visible changebut will interact with the yellow particles 10 a in cell 14 and with thecyan particles 10 b in cell 15. As a result of interacting with theyellow and cyan particles, light reflecting from the pixel 26 in FIG. 3will appear green to the viewer.

A similar situation is illustrated in FIGS. 4 and 5. In FIG. 4, cell 14is in its collected state while cells 15 and 16 are in their distributedstate. Ambient light will pass through cell 14 without significantchange but will interact with the cyan particles 10 b in cell 15 andwith the magenta particles 10 c in cell 16. As a result of interactingwith the cyan and magenta particles, light reflecting from the pixel 26in FIG. 4 will appear blue to the viewer. In FIG. 5, cell 15 is in itscollected state while cells 14 and 16 are in their distributed state.Ambient light will pass through cell 14 without significant visiblechange but will interact with the yellow particles 10 a in cell 14 andwith the magenta particles 10 c in cell 16. As a result of interactingwith the yellow and magenta particles, light reflecting from the pixel26 in FIG. 5 will appear red to the viewer.

Therefore, by selecting the appropriate combination of two cells in adistributed state with one cell in a collected state, the pixel 26 cangenerate any one of the three primary colors. By selecting combinationscomplementary to these, that is, two cells in a collected state with onecell in a distributed state, the pixel 26 can generate any one of thethree subtractive primary colors. This is illustrated in FIGS. 6 through8.

In FIG. 6, cell 16 is in its distributed state, while cells 14 and 15are both in their collected state. Ambient light passes through cells 14and 15 without significant visible change but interacts with the magentaparticles 10 c in cell 16. As a result of interacting only with themagenta particles, light reflecting from the pixel 26 in FIG. 6 willappear magenta to the viewer. In FIG. 7, cell 14 is in its distributedstate while cells 15 and 16 are in their collected state. Ambient lightpasses through both cells 15 and 16 without significant visible changebut interacts with the yellow particles 10 a in cell 14. Lightreflecting from the pixel 26 in FIG. 7, therefore, will appear yellow tothe viewer. In FIG. 8, cell 15 is in its distributed state, while cells14 and 16 are in their collected state. Ambient light passes throughcells 14 and 16 without significant visible change but interacts withthe cyan particles 10 b in cell 15. As a result, light reflecting fromthe pixel 26 in FIG. 8 will appear cyan to the viewer.

The process of constructing a preferred embodiment of the stackedelectrophoretic display of the present invention can be followed byreference to FIG. 9, a sectional view illustrating the structure of asingle pixel 26 in a reflective electrophoretic display device withthree stacked color cells.

Each pixel 26 has three separate solid state switches or drivingelements 3 a, 3 b, and 3 c, each of which is preferably a thin filmtransistor or a metal-insulator-metal device, formed on the top surfaceof rear window 4 c of cell 16 near the lateral center of the pixel 26.Driving element 3 a is used to operate post electrode 20 a in cell 14,driving element 3 b is used to operate post electrode 20 b in cell 15,and driving element 3 c is used to operate post electrode 20 c in cell16. The driving elements 3 a, 3 b, and 3 c and their associatedconnections preferably occupy a small fraction of the total lateral areaon the top surface of the rear window 4 c of cell 16.

A white reflective layer 6 covers the bottom surface of the rear window4 c of cell 16. The white color can be achieved by using a metalliclayer such as aluminum or chromium in conjunction with a light-diffusingover layer, or can be achieved by using a layer of properly-sized highrefractive index particles properly suspended in a low refractive indexmedium, i.e. white paint. A transparent insulating film 5, such as ofSiO₂, covers the top surface of the rear window 4 c of cell 16,including the driving elements 3 a, 3 b, and 3 c and their associatedconnections. To make electrical contact between the driving elements andtheir respective electrodes, common lithographic and etching techniquescan be used to create properly aligned holes through the insulating film5.

Standard lithographic, etching, and deposition techniques (for exampleas described in IBM's U.S. Pat. No. 6,144,361) can be used to create thewall electrode 8 c, the vertical wires 7 a and 7 b that reside insidethe post electrode 20 c, and the post electrode 20 c itself. The postelectrode 20 c is formed directly on the driving element 3 c through itscontact hole in the insulating layer 5. Vertical wires 7 a and 7 b areformed directly on the driving elements 3 a and 3 b respectively, andallow electrical signals originating from their respective drivingelements to pass through cell 16 on their way to post electrodes 20 aand 20 b, respectively. Plates 2 b, 2 c have holes that permit thepassage of electrical conductors from the driving elements on thesurface of the rear panel to the counter electrodes in each of thecells. The holes may be filled with electrically conductive materialthat serve as the conductors connecting the vertical wires, as shown inFIG. 9, for example, for the ends of wire 9 a that are in contact withthe conductive material in the holes of windows 2 b, 2 c.

To facilitate filling of cell 16, a second lithographic/etching step maybe performed to create at least one notch 11 c on each side of the topsurface of the wall electrode 8 c. The top of cell 16 is formed byplacing a thin transparent plate on the top surfaces of the wallelectrode 8 c, the post electrode 20 c, and the vertical wires 7 a and 7b. This plate acts as both the top surface 2 c for cell 16 and thebottom surface 4 b for cell 15. A top-down view of cell 16 at this stageof construction is illustrated in FIG. 10.

The next level of construction begins by using lithographic and etchingtechniques to create holes in the thin plate 2 c/4 b that expose andallow connection to the vertical wires 7 a and 7 b. Standardlithographic, etching, and deposition techniques can be used to createthe wall electrode 8 b, the vertical wire 9 a that resides inside thepost electrode 20 b, and the post electrode 20 b itself. The postelectrode 20 b is formed directly on the vertical wire 7 b (that isconnected to driving element 3 b). Vertical wire 9 a is formed directlyon vertical wire 7 a and allows electrical signals from vertical wire 7a (that originate from driving element 3 a) to pass through cell 15 ontheir way to post electrode 20 a.

To facilitate filling of cell 15, a second lithographic/etching step maybe performed to create at least one notch 11 b on each side of the topsurface of the wall electrode 8 b. The top of cell 15 is formed byplacing a thin transparent plate on the top surfaces of the wallelectrode 8 b, the post electrode 20 b, and the vertical wire 9 a. Thisplate acts as both the top surface 2 b for cell 15 and the bottomsurface 4 a for cell 14. A top-down view of cell 15 at this stage ofconstruction is illustrated in FIG. 11.

The post electrodes 20 c and 20 b are hollow and thus have passages forelectrical connectors, such as the wires 7 a, 7 b and 9 a, which arenested within the electrodes 20 c and 20 b. Nesting wires 7 a and 7 binside the hollow post electrode 20 c, and nesting wire 9 a inside thehollow post electrode 20 b, permit electrical connection to upper postelectrode 20 a while the surrounding electrodes 20 c and 20 b shield thesuspension in the lower cells from the electric field generated by thenesting wires 7 a and 7 b. In this way, the electrophoretic suspensionin any upper cell can be switched between its collected and distributedstates with reduced influence on the suspension associated with any cellbelow it.

The last level of construction begins by using lithographic and etchingtechniques to create the holes in the thin plate 2 b/4 a that expose andallow connection to the vertical wire 9 a. Standard lithographic,etching, and deposition techniques can be used to create the wallelectrode 8 a and the post electrode 20 a. Post electrode 20 a is formeddirectly on vertical wire 9 a, which is connected to vertical wire 7 a,which in turn is connected to driving element 3 a.

To facilitate filling of cell 14, a second lithographic/etching step maybe performed to create at least one notch 11 a on each side of the topsurface of the wall electrode 8 a. The top of cell 14 is formed byplacing a thick transparent plate on the top surfaces of the wallelectrode 8 b and the post electrode 20 b. This plate acts as both thetop surface 2 a for cell 15 and as the top surface for the pixel 26. Atop-down view of cell 14 at this stage of construction is illustrated inFIG. 12.

The wall electrodes 8 a, 8 b, and 8 c for every pixel 26 in the displayare preferably held at a common voltage, which is preferably ground. Toensure that the three wall electrode structures (one associated witheach of the three layers) are held at a common voltage, an electricalconnection can be made between the outside edges of the outermost pixelsof the display, across the thin transparent plates 2 c/4 b and 2 b/4 a.Alternatively, using standard lithographic, etching, and depositiontechniques, an electrical connection between the three wall electrodestructures could be formed through holes in the thin transparent plates2 c/4 b and 2 b/4 a. Since this grounded electrode wall structurelaterally surrounds the post electrode and the electrophoreticsuspension in every subpixel, and since the wall structures in eachlayer are vertically aligned and in electrical contact, stray electricfields that can adversely influence the suspension in neighboringsubpixels are substantially reduced.

The different layers in a pixel 26 possess suspensions with differentlycolored pigment particles, while all the cells in a given layer possessthe same color of pigment particles. Suspensions with the desiredcolored particles can be introduced to each of the three layers byseparately filling each layer via capillary and/or vacuum action throughthe notches 11 a, 11 b and 11 c in the outside edges of the outermostpixels of the display. U.S. Pat. No. 5,279,511 assigned to Copytele Inc.describes an alternative process for filling a single cell in anelectrophoretic display. Using this process, the three differentlycolored dry pigment particles 10 c, 10 b, and 10 a are coated onto thebottom, middle, and top layers respectively, at the appropriate timeduring the construction of each layer. A common light-transmissivesuspension fluid 12 can then be simultaneously introduced to all threelayers via capillary and/or vacuum action through the notches 11 in theoutside edges of the outermost pixels of the display.

Other embodiments of this invention can use different color combinationsfor the cells within the pixel, can use a different number of cells foreach pixel, or can use selectively colored reflective panels. Eventhough it would reduce the color gamut, red, green, and blue particles,for example, could be used instead of cyan, magenta, and yellowparticles. Reducing the number of cells in the vertical stack of thepixel from three to two would also reduce the color gamut, but couldsimplify the construction and/or operation of the device. Increasing thenumber of cells in the vertical stack could also be advantageous. Addinga fourth cell to the stack that contained black absorbing particles, forexample, could enhance pixel blackness while in the distributed stateand improve the contrast of the display. Even through it would reducethe color gamut, a colored reflective panel could be used in conjunctionwith white (scattering) particles in the bottom-most cell.

Other embodiments of this invention can also use a different number ofcollecting and/or counter electrodes, and their positions and/or shapesand/or sizes can be different from that described. The distributed stateneed not be one in which the particles are distributed throughout thevolume of the cell, the particles could be forced to form a layer acrossone or more horizontal surfaces of the cell. In addition, somecomponents in the illustrations above may not be necessary or could bemodified in other embodiments.

Although this invention has been described with respect to specificembodiments, the details are not to be construed as limitations for itwill be apparent that various embodiments, changes, and modificationsmay be resorted to without departing from the spirit and scope thereof.Further it is understood that such equivalent embodiments are intendedto be included within the scope of this invention.

What is claimed is:
 1. A color electrophoretic display comprising: asubstantially planar light-reflective rear panel; a substantially planarlight-transmissive front window generally parallel to and spaced fromthe rear panel; a plurality of pixels laterally adjacent to one anotherand located in the space between the rear panel and the front window,each pixel comprising multiple cells stacked from the rear panel to thefront window, each cell in the stack containing charged, light-absorbingpigment particles in a light-transmissive fluid, and all laterallyadjacent cells from different pixels forming layers of cells with eachlayer having pigment particles of the same color; a collecting electrodesubstantially non-obstructing to light and associated with each cell tosubstantially remove pigment particles in the associated cell from thepath of light through the cell; and a counter electrode substantiallynon-obstructing to light and associated with each collecting electrodeto distribute the pigment particles throughout the light-transmissivefluid of the associated cell so as to be in the path of light throughthe cell.
 2. An electrophoretic display as claimed in claim 1 furthercomprising a first light-transmissive plate generally parallel to therear panel and separating the cells in the layer nearest the rear panelfrom cells in the neighboring layer, and wherein the first plateincludes a plurality of holes, each hole associated with a cell, forpermitting electrical connection from the rear panel to the electrodesin the cells in said neighboring layer.
 3. An electrophoretic display asclaimed in claim 2 further comprising side walls substantiallyperpendicular to and between the rear panel and the first plate, theside walls having openings for permitting filling of the cells with thelight-transmissive fluid.
 4. An electrophoretic display as claimed inclaim 3 wherein the side walls are formed of electrically conductingmetal, and wherein the collecting electrode of each cell in the layer ofcells nearest the rear panel comprises a metal side wall.
 5. Anelectrophoretic display as claimed in claim 2 wherein the collectingelectrode of each cell in the layer of cells nearest the rear panelcomprises a film of electrically conductive material deposited on theside walls.
 6. An electrophoretic display as claimed in claim 2 whereinthe counter or collecting electrode of each cell in the layer of cellsnearest the rear panel comprises a film of electrically conductivematerial deposited on the interior wall of the hole associated with saideach cell.
 7. An electrophoretic display as claimed in claim 1 furthercomprising a solid state switch associated with each cell and located onthe rear panel, each switch being electrically connected to the counterelectrode of the associated cell.
 8. An electrophoretic display asclaimed in claim 1 wherein there are three cells in a stack, and whereinthe pigment particles in the three stacked cells are colored magenta,cyan and yellow, respectively.
 9. A reflective color electrophoreticdisplay comprising: a light-reflective rear panel having a substantiallyplanar horizontal surface; a substantially planar light-transmissivefront window generally parallel to and spaced from the horizontalsurface of the rear panel; first and second substantially planarlight-transmissive plates located between the rear panel and frontwindow and generally parallel to the horizontal surface of the rearpanel; a plurality of vertical side walls for spacing the first platefrom the horizontal surface of the rear panel, the second plate from thefirst plate, and the front window from the second plate, the verticalside walls defining a plurality of stacks of three vertically-alignedcells, each cell in a stack containing a light-transmissive fluid andcharged, light-absorbing pigment particles having a color different fromthe color of the pigment particles in the other two cells in the stack,each stack thereby forming a multicolored pixel; an electrode on thecell-interior surface of the vertical side walls for collecting thecharged pigment particles to substantially remove them from the path oflight through the cell; a counter electrode in each cell fordistributing the pigment particles throughout the light-transmissivefluid of the cell to place the particles in the path of light throughthe cell; and a plurality of counter electrode driving elements on thehorizontal surface of the rear panel, each driving element beingassociated with and electrically connected to a counter electrode. 10.An electrophoretic display as claimed in claim 9 wherein the first plateincludes a plurality of holes, each hole associated with a cell, forpermitting electrical connection from the driving elements on the rearpanel to the counter electrodes in the cells between the first andsecond plates.
 11. An electrophoretic display as claimed in claim 10wherein the second plate includes a plurality of holes, each holeassociated with a cell, for permitting electrical connection from thedriving elements on the rear panel through the holes in the first plateand to the counter electrodes in the cells between the second plate andthe front window.
 12. An electrophoretic display as claimed in claim 11wherein the counter electrode in each of the cells nearest the rearpanel comprises an electrically conducting structure located in a holeof the first plate and having a passage, and further comprising anesting wire located in the passage and electrically connected to adriving element and to a counter electrode in a cell between the secondplate and the front window, the counter electrode in each of the cellsnearest the rear panel thereby surrounding the nesting wire andshielding the electrophoretic suspension in the cell.
 13. Anelectrophoretic display as claimed in claim 11 wherein the collectingelectrodes in each of the cells are electrically connected together,whereby all of the collecting electrodes can be maintained at a commonvoltage level.
 14. An electrophoretic display as claimed in claim 9wherein the vertical side walls have openings for permitting filling ofthe cells with the light{-}transmissive fluid.
 15. An electrophoreticdisplay as claimed in claim 9 wherein the vertical side walls are formedof electrically conducting metal and wherein the cell-interior surfaceof the vertical metal side walls is the collecting electrode.
 16. Anelectrophoretic display as claimed in claim 9 wherein the collectingelectrode of each cell comprises a film of electrically conductivematerial deposited on the cell-interior surface of the vertical sidewalls.
 17. An electrophoretic display as claimed in claim 9 wherein thedriving elements comprise thin film transistor devices.
 18. Anelectrophoretic display as claimed in claim 9 wherein the drivingelements comprise metal-insulator-metal devices.
 19. An electrophoreticdisplay as claimed in claim 9 wherein the pigment particles in the threecells in each stack are colored magenta, cyan and yellow, respectively.